1. Field of the Invention
This invention relates to a method and apparatus for measuring the filling effectiveness of the filling operation in the manufacture of waterproof cables, wherein the capacitance between an outer pair of conductors is measured and the capacitance between an inner pair of conductors is measured, and the capacitances are compared.
2. Description of the Prior Art
Telecommunications cables, especially those that are to be buried in the ground, are desirably moisture proofed to prevent transmission difficulties resulting from the seepage of moisture into the cable. In general, such moisture proofing is accomplished during manufacture of the cable by filling the internal volume of the cable with a suitable filling compound, such as, for example, petrolatum or a mixture of petrolatum and polyethylene. For the desired results to be achieved, the filling material should preferably occupy substantially all of the volume of the cable that is unoccupied by the conductors and other components therein, including the interstices between twisted pairs of conductors. Various methods and apparatus for filling cables are shown, for example, in U.S. Pat. Nos. 3,832,215, 3,854,444, and 3,850,139 of Franke et al., 3,789,099 and 3,876,487 of Garrett et al., 3,733,225 of Moody, and in copending U.S. patent application Ser. No. 457,877 of Freeman et al., filed Apr. 4, 1974, and assigned to the present assignee, now abandoned.
The normal filling procedure involves the introduction of the filling compound after the core has been formed and before the final binder and sheath are placed on the core. At this stage of manufacture, the core is relatively compact and it is difficult to introduce the filling compound, yet the prevention of the ingress of moisture in subsequent use requires that there be a high percentage of fill in the total fillable volume, preferably evenly distributed throughout the cable cross-section.
Numerous arrangements for ascertaining the amount or percentage of fill material in a cable, which is an indication of filling operation effectiveness, have been devised. One such arrangement comprises cutting off an end portion of a finished cable and subjecting one end thereof to water under a known pressure. If more than a predetermined amount of water flows out of the other end, the cable is unacceptable. Another arrangement comprises weighing a short length of filled cable. Since the unfilled weight is known, and the weight of the proper amount of fill material for such a length can be determined, the weight of the filled length of cable should at least equal the sum of the two to be acceptable.
Still another method for determining the acceptability of the filled cable comprises measuring the capacitance of a number of pairs of outer conductors in a finished cable, then measuring the capacitance of a number of pairs of inner conductors, and comparing the two measurements. The difference between the two measurements, divided by the outer measurement provides a measure of the filling effectiveness which can then be compared to empirically predetermined values to ascertain whether or not the cable is acceptable.
In the prior art methods of determining filling effectiveness, examples of which are given in the foregoing, the operations are performed on a finished cable, hence if the filling effectiveness is found to be inadequate, a whole cable run must be scrapped or attempts made to refill the cable. In those processes where the measurements or tests are made on a short length of cable, there is no way of determining whether the remainder of the cable is the same as the tested sample, hence a calculated risk is taken in depending on the test results. In those arrangements where the entire cable length is tested, as in the capacitance measuring method, an indication of non-acceptability may result from only a very short faulty length of cable, which could be cut out if its location along the cable length were known.
This latter problem is common to virtually all of the prior art arrangements, namely, there is no way of ascertaining where, along the cable length, the amount of fill has fallen below an acceptable minimum. An additional drawback of prior art testing methods is that they are performed on finished cables, and unacceptable cables must be scrapped or refilled, which entails both extra time and money.